1. Field of the Invention
The present invention relates to a bead wire for a bead core of a tire and to a tire incorporating the same, and more particularly to a bead wire for a bead core of a tire designed to allow easy mounting of the tire on a wheel rim and also designed to resist the separation of the tire from the rim under diverse operating conditions.
2. Description of Related Art
As is known in the art, a tire generally comprises at least one carcass reinforcing ply having edges turned up around bead cores, a tread, and belts placed between the carcass and the tread. Strips of rubber filling are positioned between the sides of the carcass reinforcing ply and the upturned edges of the same reinforcing ply. The portion of the tire comprising the bead core and the rubber filling forms the tire beads, which function to anchor the tire over a corresponding mounting rim.
The wheel rim comprises a central cylindrical channel from which branch off axially and outwardly, from opposite sides, diverging surfaces each terminating in a vertical peripheral flange called the “rim balcony”. The diverging surfaces form the bead seats for the beads of the tire.
The bead core is substantially inextensible in circumferential direction and can be formed of a single element or of several elements, such as steel wires, steel cords, and other similar components placed in an annular disposition. The individual elements are referred to as bead wires, and when assembled together they form the bead core.
The inner diameter of the tire beads substantially coincides with the diameter of the innermost surface of the bead cores, except for a difference between the two diameters caused by a thin lining of rubber. The diameters of the inner annular surfaces of the bead cores and tire beads are smaller than the diameter of the rim balcony, and are chosen so that, once the beads are forced over the balcony to their respective bead seats on the rim, they are pushed along the diverging surfaces of the bead against the inner surfaces of the flanges by the pressure of the air inside the tire.
The operations of mounting the tire onto the rim are performed according to methods well known to those skilled in the art. The operation starts by deforming the first bead of a tire into an oval configuration, so that when positioned in front of the rim with the oval aperture suitably oriented, a portion of the bead slips over the balcony of the rim. Then the rest of the bead completely slips over the rim balcony, so that the bead can then be pushed toward the bead seat. The preceding steps are then repeated for the second bead. Finally, the tire is inflated to press the beads against the internal surfaces of the rim balcony of the bead seat.
Because of the rigidity of the bead cores, mounting the tire on the rim requires application of a large force to deform the bead core from its circular configuration to an oval one, causing obvious difficulties of application for certain bead core structures, and with a risk of breakage in some circumstances if the limits of elastic deformability of the bead wires are exceeded.
Since the pressurized air in the tire is used to maintain the tire bead pressed against the inner surface of the rim flange, when the tire deflates this force is no longer applied, and the tire bead can leave its bead seat falling into the central channel of the rim. This unseating, usually, immobilizes the vehicle because the rim balcony quickly enters in contact with the pavement, making traction and control of the vehicle impossible. For example, a conventional tire is inflated at an operating pressure of 1.8–2.0 bars. When the inflation pressure falls below approximately 0.8 bars, the tire beads are likely to unseat from the bead seats. As a result, a second requirement for modern tires is to be able to remain in place on the rim even in the event of a perforation and subsequent loss of air within the tire. This requirement calls for a bead core exerting sufficient force on the rim so that the tire beads will remain seated on the rim, even in the absence of air pressure pushing them in place. This result cannot be achieved using conventional bead cores, unless humps are formed on the rim to act as a barrier preventing the beads from slipping in the center channel portion of the rim. According to the state of the art, it has not been possible to provide bead cores that exert sufficient force on the rims to maintain the tire beads on the rim when the tire deflates, and that at the same time can be stretched sufficiently to allow mounting of each tire on the rim with conventional tools.
Several types of conventional bead cores used in tire beads are known. For example, a first design provides for a bead core formed by a rubber-coated steel wire wound in a spiral to form a first layer of side-by-side coils. Subsequent layers are superposed on the first layer, and also consist of helical windings of the same wire. One known construction of this type of bead core comprises four layers of four wires each. An additional type of bead core calls for the use of several individual wires and, more precisely, of a first wire wound in a spiral to form several coils arranged radially in a vertical plane. Subsequent wires are similarly wound in vertical planes, and are placed side by side to the first plane. One such construction known as 4×4 comprises four wires disposed in four layers.
Bead cores with improved characteristics of flexibility, and consequently greater deformability without risk of breakage, are also known. One of these known structures is a spiral bead core. This bead core is formed of a central cable around which several wires are wound in a spiral. In this design, 4 or 5 groups of steel wires can be disposed in a structure having 4 or 5 layers of the steel wire groups, set up in a spiral pattern. However, the construction of this spiral bead core requires a number of separate reels for the cable and for the various wires. This results in increased manufacturing costs than, for example, those incurred in the construction of a bead core containing a single type of wire.
Another problem encountered in mounting tires is that generally, in the construction of the bead cores as well as in the construction of the wheel rims, the actual dimensions obtained often vary from their selected tolerances. In these cases, dovetailing of the tire bead over the relative bead seats can occur. This may result in breaking of the bead core when the actual diameter of the tire bead is significantly smaller than the specified dimension, or in slipping of the tire bead over the bead seat during rotation when the bead core is larger than the specified dimension.
German Patent Application DE 3829460 A1 describes a bead core made with a shape-memory material, preferably a Ni—Ti alloy. The mounting method described in the application calls for temporarily deforming the bead core into an oval configuration, and after mounting the tire on the rim, submitting the bead core to heat treatment at the crystallization temperature of the alloy (between 65 Deg. C. and 90 Deg. C. ), so that the bead core recovers its annular shape. Thus this method provides a bead core that holds the tire beads against the rim with sufficient force while being elastic enough to allow mounting of the tire. However, the mounting of the tire according to this method requires specialized tools and heating equipment.
Known practices to resist tire unseating are generally based on modifying the surfaces of contact of both the tire beads and the bead seats. One of these practices entails using a protuberance on the base of the tire bead and an aperture on the rim designed to receive the protuberance. The protuberance of the tire bead when inserted into the aperture of the rim prevents the tire beads from separating from the rim.